Imaging process



Aug. 12, 1 969 w J- K. SWIGERT ET AL IMAGING PROCES S Filed Dec. 27, 1965 FIG] INVENTQR. RICHARD L. LANE J Y KIRK sw/amr .grrofivsrs United States Patent s. or. 101-450 Claims This invention relates to an imaging system, and more specifically, to lithography.

Lithographic printing is a well known and established art. In general, the process involves printing from a flat plate, depending upon different properties of the image with the non-image areas for printability. In conventional lithography, the non-image area is hydrophilic while the image area is hydrophobic. In the lithographic printing process, a fountain solution is applied to the plate surface which wets all portions of the surface not covered by the hydrophobic image. This solution keeps the plate moist and prevents it from scumming up. An oil based printing ink is applied to the image surface depositing the lithographic ink only on the image area, the hydrophilic nonimage repelling the ink. The ink image may then be transferred directly to a paper sheet or other receptive surface, but generally it is transferred to a rubber offset blanket which in turn transfers the print to the final paper sheet. Hence, for each print made during a run, a lithographic plate is first dampened with an aqueous fountain solution and then inked with a lithographic ink and finally printed.

It has been known that lithographic plates can be made in a photoconductive system by utilizing the conventional developed xerographic plate as a lithographic printing plate. In these systems, usually a zinc oxide type plate is charged by conventional means exposed to the image to be reproduced, and developed with conventional xerographic toner. The toner used to develop the image is generally hydrophobic in nature as is the background, nonimaged areas of the conventional binder type xerographic plate. In order that the developed xerographic plate be useful as a lithographic master, a differential must be established between the tonered image and the background of the plate. Since both are hydrophobic in nature, it has heretofore been required that the background of the xerographic plate be treated, either by the use of a conversion solution, to make it hydrophilic in nature or by removal with a selective solvent. After the alteration of the background, the plate is then wetted with a non-aqueous or oil based ink whereby the toner will accept the ink and the now hydrophilic background will repel the ink.

While basically these systems have been found useful for lithographic purposes, there are inherent disadvantages to their use. One disadvantage, for example, is the fact that it is required that secondary solutions be used to convert the initially hydrophobic background so that it will not accept the oil based ink in the inking step. A second disadvantage to these systems is that the xerographic plate, when developed, has toner fused thereon, fixed to the surface of the plate and inherently non-reusable. A further disadvantage is that once the image is developed and fused, it generally can no longer be conveniently altered once printing has commenced, in order to enhance the quality of the desired reproduction.

It is, therefore, an object of this invention to provide a lithographic imaging system which will overcome the above noted disadvantages.

It is a further object of this invention to provide a novel method for the preparation of a lithographic master plate.

Another object of this invention is to provide an imaging system utilizing a lithographic master prepared from a xerographic plate.

Still a further object of this invention is to provide an imaging system wherein the novel lithographic plate can be reused.

Yet, still a further object of this invention is to provide an imaging system utilizing a lithographic master prepared by a one-step process.

An additional object of this invention is to provide a novel lithographic plate wherein both sides of the plate may serve as the printing surface.

The foregoing objects and others are accomplished in accordance with this invention, generally speaking, by providing a xerographic plate prepared by fixing to the surface of a receiving substrate a glass photoconductive insulating layer. The layer can then be electrostatically charged and imaged in accordance with the conventional xerographic imaging process more fully described in US. Patent 2,297,691. The plate of the present invention is generally prepared by intimately mixing a photoconductive material in a glass binder insulating composition and fixing the resulting photoconductive blend to a conductive backing to form a substantially uniform layer comprising photoconductive particles embedded in a glass binder composition. The electrostatic image is developed with a hydrophobic developer and the resulting developed image fixed so as to produce a duplicating plate having both hydrophobic and hydrophilic properties. The lithographic master can then be used in a one-step process to continuously make prints whereby a lithographic ink is applied to the master, the ink adhering; only to the hydrophobic tonered image and the master subsequently contacted with a copy sheet to transfer the image. It is to be understood, that if it is desired to develop the electrostatic latent image of the present invention with a hydrophilic developer, the background, non-imaged area of the plate may be treated in such a manner so as to exhibit oleophilic properties. If it is desired to reuse the master plate the tonered image may be removed from the surface of the plate in a manner which does not have a detrimental effect upon the photoconductive layer of the late.

p In lieu of the above method of preparing the glass photoconductive xerographic plate of this invention, it is possible to dissolve a photoconductive material in a glass insulating composition in such a manner so as to produce a homogeneous glass photoconductive plate. The method of preparing such a homogeneous glass photoconductive plate is further described in application Ser. No. 516,726, concurrently filed with the instant case and having a common assignee.

It has been found, in accordance with the present invention, that when the photoconductive material is dispersed within a glass binder formulation and the resulting composition is bonded to the surface of a suitable substrate that the resulting photoconductive plate inherently possesses the necessary properties required for a lithographic duplicating system. That is, the photoconductive insulating layer comprising the glass binder composition of this invention is hydrophilic in nature and therefore, already having the necessary hydrophilic properties need not be treated to establish a differential between the non-image and image areas of the lithographic master, a procedure which is known to be required by the conventional lithographic plates as taught in US. Patents 3,107,169 and 3,001,872. Furthermore, if it is desired to reuse the plate of this invention, due to the stability of the glass binder composition, it is possible to remove the tonered image from the surface of the plate without destroying the properties of the underlying photoconductive layer. This procedure is generally accomplished with a suitable solvent. It has been determined that the use of these solvents, generally, would detrimentally effect the hydrophobic binder compositions now presently used in the preparation of lithographic plates.

The invention is illustrated in the accompanying drawings in which:

FIG. 1 represents a magnified cross section through a lithographic printing plate having a two-phase photoconductive insulating layer;

FIG. 2 represents a magnified cross-section through a lithographic printing plate having a homogeneous, single phase photoconductive insulating layer.

In the present process for preparing a lithographic plate 1 as illustrated in FIG. 1, a suitable substrate 2 is coated with a photoconductive insulating composition 3 comprising a photoconductive material 4 and a glass binder composition 5. On the surface of the photoconductive layer is superimposed a developed toner image 6. The selection of the supporting substrate 2 is based upon the desired use of the lithographic plate, such as to give the plate additional strength or to provide added flexibility in situations requiring it. FIG. 2 illustrates a lithographic plate 7 wherein the photoconductive insulating layer 9 is a homogeneous, single-phase glass composition. The. substrate 8 and developed image 10 are similar to the substrate 2 and image 6 of FIG. 1.

In accordance with the present invention, a glass binder insulating composition is blended with a photoconductive material in proportions of from about to about 45 parts of the photoconductive material to about 100 parts of a glass binder composition. Preferably, it has been found that about 8 to about 25 parts photoconductive material to about 100 parts glass binder composition will give optimum results. The finely divided photoconductive particles and glass binder particles are desirably dispersed in a liquid, such as distilled Water, or an organic liquid, such as ethylene glycol, by any suitable means, such as in a ball mill. The photoconductive material and the glass binder composition may be ground together in water using small amounts of sodium silicate, sodium hydroxide, and boric acid as dispersing agents. The resulting slurry is thoroughly dried to remove most of the solvent and then blended with an Organic liquid such as isopropyl alcohol. The milling step is repeated to produce the final photoconductive coating composition. The iesulting composition is then applied to the desired surface by flow coating, dipping, spraying, electrostatically, with a doctor blade or by any other sutiable coating operation. Care must be taken that air bubbles or other discontinuities are eliminated from the slurry before coating to insure good adhesion to the underlying substrate and to retain good photoconductive properties. The coating is dried to remove most of the liquid and the plate subsequently fired at the necessary temperature to fuse and bond the glass binder composition to the base substrate and produce a uniform layer of the photoconductive pigment dispersed in the glass binder composition. The firing temperature is such that it is suflicient to fuse the glass binder composition and, therefore, will vary with the particular binder composition chosen. An applicable temperature range is found to be about 300 to 1500 F. again, depending upon the gass binder composition.

The glass binder composition material should be so selected relative to the photoconductive material so that the melting point of the binder is lower than that of the photoconductive material and also the base material. Thus, the maximum melting temperature permissible in the binder will vary depending upon the photoconductive material selected.

The length of time necessary to fire the glass binder photoconductive composition in order to fuse and bond it to the underlying substrate will be dictated by the glass frit formulation chosen. Preferred firing time ranges from about 1 to 60 minutes depending upon the materials used, in order to obtain optimum electrical characteristics. The support layer should be cleaned prior to coating thereon. Any suitable cleaning technique may be used, such as the application of organic solvents. Aluminum layers, for example, may simply be heated to firing temperature and cooled before the coating material is applied.

The resulting glass binder photoconductive plate is electrostatically charged in the dark and the charged surface selectively exposed to a light source to produce a latent image. The imaged surface is next developed with hydrophobic electroscopic particles or toner, the developer adhering to the areas corresponding to the latent image, and the resulting tonered image fused by heating the plate to a temperature of about 200500 F. for about 2-12 minutes. It is also possible to develop the electrostatically charged image with a liquid developer containing charged hydrophobic particles suspended in a carrier. The photoconductive plate is then fastened to a press cylinder of a lithographic press. The printing operation is carried out utilizing commercially available fountain solutions and lithographic inks. The tonered particles used to develop the electrostatic latent image may consist of a pigmented resin which is hydrophobic in nature and which may be readily wetted by a lithographic ink.

The printing surface of the glass binder photoconductive plate of this invention is such that, when dry, it will readily accept a water-repellant image which adheres tenaciously thereto and shall neither be pulled away by printing ink nor washed away by the wet-out or fountain solutions. Furthermore, the non-image areas of the plate are readily wetted by the fountain solutions and hold a film thereof on the surface of the non-image areas and do not permit the aqueous film to be displaced therefrom by the lithographic printing ink.

If it is desired to reuse the photoconductive plate of this invention, the plate is cleaned by removing the tonered image with a selective solvent, such as methyl ethyl ketone, or acetone. Generally, the commercially available acidic fountain solutions utilized in the course of this invention are neutralized with a basic material, such as sodium hydroxide, to increase the pH of the solution and to reduce etching. However, if a slight etch of the glass binder plate does result, the glass binder photoconductive plate can be refired at a temperature and for a time suflicient to regenerate the plate. Regeneration of the plate is generally accomplished at a temperature of about 200l100 F. for a time of about 1-10 minutes, depending upon the materials used.

In general, any suitable glass binder composition which displays wettable hydrophilic properties may be used in the course of the instant invention. This includes both photoconductive and non-photoconductive glass binder insulators. Typical such glass binder compositions are borosilicates, alkali silicates, lead silicates, lead alumino phosphates, soda-lime silicates and Pyroceram Glass No. and No. 89, glass formulations available from the Corning Glass Company. Other usable glass binder materials are disclosed in U.S. Patent 3,151,982. Preferred are those glass frits which have a fusing temperature at the lower end of the acceptable temperature range. These glass binder materials have been found to possess substantial amounts of boric oxide and lead oxide. Sodium and potassium oxide also lower the fusing point of the glass frit, but must be limited to prevent water solubility. Fluorides also lower melting temperature but cause silica and boric oxide solubility. Calcium oxide, and especially zinc oxide and cadmium oxide lower the melting point to a certain degree. Antimony and arsenic oxides also lower the melting point of the glass binder material.

The photoconductive material should be in a suitable, finely divided state. While photoconductive particle sizes as large as about 50 microns may be used, it is preferred that particle size be as small as possible in order to obtain optimum results. In general, particle sizes of no more than about 20 microns are used, preferably the photoconductive particles should have an average particle size of no more than about one micron.

Any suitable photoconductive material may be used in the course of this invention. Typical such inorganic photoconductive materials are sulfur, selenium (vitreous, amorphous, monoclinic) zinc sulfide, zinc oxide, zinc cadmium oxide, zinc magnesium oxide, cadmium selenide, zinc silicate, calcium-strontium sulfide, cadmium sulfide, mercuric iodide, mercuric oxide, mercuric sulfide, indium trisulfide, gallium triselenide, arsenic disulfide, arsenic trisulfide, arsenic triselenide, antimony trisulfide, cadmium sulfoselenide, doped chalcogenides of Zinc and cadmium, aluminum oxide, bismuth oxide, molybdenum oxide, lead oxide, molybdenum iodide, molybdenum selenide, molybdenum sulfide, molybdenum telluride, aluminum iodide, aluminum selenide, aluminum sulfide, aluminum telluride, bismuth iodide, bismuth selenide, bismuth sulfide, bismuth telluride, cadmium telluride, mercuric selenide, mercuric telluride, lead iodide, lead selenide, lead sulfide, lead telluride, cadmium arsenide, lead chromate, gallium sulfide, gallium telluride, indium sulfide, indium selenide, indium telluride, red lead, and mixtures thereof. Particularly, preferred materials in order to obtain optimum results are appropriately doped chalcogenides of zinc and cadmium, more specifically, the sulfides and selenides of these metals either as mixed sulfides and selenides of zinc and/or cadmium or as a mixed zinc and cadmium sulfide or selenides, simple compounds of zinc and cadmium sulfide, cadmium sulfoselenide, and mixtures thereof. Any suitable organic photoconductive material which is capable of withstanding the temperature requirements of the present invention may be used in the course of this invention. Typical such materials are the phthalocyanine pigments.

The thickness of the photoconductive insulating layers of the instant invention may vary from about 5 microns to about 200 microns. It is preferred that the layers be from about 10 to about 150 microns thick in order to achieve optimum results. The photoconductive insulating layers of the present invention are characterized by outstanding wear-resistance properties. The desired thickness may be built up by multiple coatings, if necessary.

Any suitable material may be used as the substrate for the lithographic master of this invention. The base or substrate used in preparing lithographic binder plates according to this invention provides physical support for the photoconductive insulating layer and also should have an electrical resistance less than the photoconductive layer so that it will act as a ground when the electrostatically charged coating is exposed to light. Typical such materials are metal surfaces such as aluminum, brass, steel, e.g. stainless and low carbon, copper, nickel, zinc, alloys and mixtures thereof. Other materials having electrical resistances similar to the aforementioned can also be used as the backing material to receive the photoconductive layer thereon. The material must, however, be capable of withstanding the temperature requirements for fusing the glass binder plate and have a coefficient of thermal expansion compatible with the particular glass binder composition used. In order to reduce or eliminate the warpage problems which may be encountered as a result of the differences in coefficient of thermal expansion mentioned above the base substrate may be coated on both sides with the glass binder photoconductive compositions of the present invention. When this practice is followed, it then becomes possible to image on either sides of the coated plate thus providing a printing plate with lithographic properties both sides of which may serve as the printing surfaces.

Any suitable wet-out or fountain solution may be used in the course of this invention. Typical such fountain solutions are 1% solutions by volume of gum cellulose and water, gum arabic and water, glycerol and water and isopropyl alcohol and water. Distilled water may also be used as the wet-out solution. Other suitable fountain solutions are disclosed in US. Patent 3,107,169. The fountain solutions may contain other constituents, such as for example, formaldehyde, and if glycerin is not present, it

may be added primarily to take advantage of its hydroscopic nature and hence by absorption of water prolong the period during which the hydrophilic surface of the lithographic plate retains its hydrophilic properties. The formaldehyde additive will also achieve this effect. Therefore, the fountain solutions containingthe glycerin or formaldehyde additive or mixtures thereof are preferred inasmuch as the resultant lithographic plate can be kept before use for relatively long periods of time as compared to those plates treated with a fountain solution not containing the respective additive. Solutions containing gum arabic are also found to be very effective inasmuch as a lithographic plate treated with such a solution is found to retain its hydrophilic properties in the non-image areas for relatively long periods of time, after removal from the press, so that the resultant printing plate can be reused without subjecting them to an additional treatment with the fountain solution.

Any suitable toner or developer may be used in the course of this invention such as those disclosed in US. Patents 2,788,288; 3,079,342 and Re. 25,136. The toner is generally a resinous material which when fixed has hydrophobic properties and will attract oily inks. Typical such developer powders are styrene polymers, polymers of substituted styrenes, for example, Piccolastics, commercially available from the Pennsylvania Industrial Chemical Corporation, phenol formaldehyde resins as well as other resins having hydrophobic properties. Other inorganic developer powders that may be found suitable for use in the present invention are powdered zinc, powdered copper, and sulfur, among others.

The developer powder may be applied directly to the latent image or admixed with a carrier such as glass beads. Preferably, the toner is applied in the form of a mixture with magnetic particles such as magnetic iron to impart a charge to the developer powder particles triboelectrically. A developer powder is so chosen that it is attracted electrostatically to the charged image or repelled from the background area to the charged image and held thereon by electrostatic attraction. If a negative charge is applied to the photoconductive insulating material, a positive toner is applied which adheres to the negatively charged image. If the charge applied is such that the latent image retains a positive charge, then a negative toner will be applied.

As mentioned above, liquid developers may also be used in the course of this invention. Such developers are disclosed in US. Patents 2,890,174 and 2,899,335. Generally, the developer comprises a liquid combination of mutually compatible ingredients, which when brought into contact with an electrostatic latent image, will deposit upon the surface of the image in an image-wise configuration. In its simplest form, the composition comprises a finely divided opaque powder, a high resistance liquid and an ingredient to prevent agglomeration. Liquids which have been found suitable include such organic high resistance liquids as carbon tetrachloride, kerosene, benzene, tetrachloroethylene and any substituted hydrocarbon having a boiling point between about 70 and 200 C. Any of the finely divided opaque solid materials known in the art such as carbon black, talcum powder, or other pigments may be used in the liquid developer. Silica aerogel commercially available from Monsanto Chemical Company is the deagglomeration ingredient generally used. However, any conventional and/ or suitable ingredient known to be useful by the prior art in a liquid development system may be utilized.

Any suitable lithographic ink may be used in the course of this invention. Typical such lithographic inks and their properties are disclosed in Printing Ink Tech nology by E. A. Apps, Chapter 11. The inks are of the same fundamental style as good quality letter-press inks, and the simplest type consist of a pigment mixture dispersed in a lithographic varnish, a heat-bodied linseed oil. The lithographic or oil-based ink being oleophilic in nature adheres to the hydrophobic tonered image, and is repelled by the hydrophilic non-imaged areas.

To further define the specifics of the present invention, the following examples are intended to illustrate and not limit the particulars of the present system. Parts and percentages are by weight unless otherwise indicated. The examples are also intended to illustrate various preferred embodiments of the present invention.

Example I Glass frit formulation:

Percent by weight SiO 18.1 B 8.1 PbO 6 5.0 CdO 7.8 TiO .07 N320 .93

Photoconductor-cadmium sulfoselenide.

Substrate-8 mil stainless steeltype No. 430.

About 200 g. glass frit of the formulation above is blended with about 32 g. of a cadmium sulfoselenide photoconductive material in about 200 g. of water. The resulting composition is mixed in a ball mill for approximately 18 hours. The resulting blend is then thoroughly dried. Approximately 200 ml. of isopropyl alcohol is added to the dried composition and the milling process is repeated for approximately 2 hours. The resulting glass binderphotoconductive composition is then applied by a process of flow coating to the surface of a stainless steel sheet, approximately 75 microns in thickness. The coated sheet is then dried and fired to a temperature of about 1150 F. for approximately 10 minutes. The late is then charged to about 550-650 volts by means of a laboratory corotron unit powered by a high voltage power supply. The charging current is 0.1 of a milliamp at 7500 volts. A transparent positive USAF test chart is placed on the charged plate and exposed with a 75 watt photofiood lamp. An exposure of about 20 foot-candle seconds is required for the glass binder-photoconductive plate. The electrostatic latent image produced is then developed by the cascade method of developing with a positive charged hydrophobic developer material. The toner image is then fused by heating the plate to a temperature of about 400450 F. for about 6-12 minutes. Following cooling of the plate, it is wrapped on the cylinder of a lithographic printing press and operated in the conventional manner using a fountain solution composed of:

Carboxymethyl cellulose 10 Glycerol Water 500 Substrate Wetting Agent F-126 (ammonium salt of perfluorocaprylic acid available from Minnesota Mining and Mfg. Co.) 5

the pH of which is adjusted to about 7 with sodium hydroxide. A lithographic ink is then applied to the printing surface of the plate and by use of a rubber off-set blanket, runs as long as about 50,000 prints are obtained.

Example II The process of Example I is repeated excepting zinc oxide is substituted for the cadmium sulfoselenide photoconductive material. The fountain solution utilized in this instance was composed of:

M1. Formaldehyde l0 Glycerol 5 Water 500 Substrate Wetting Agent F-126 (ammonium salt of perfluorocaprylic acid available from Minnesota Mining and Mfg. Co.) 5

Runs as long as 55,000 prints are obtained,

8 Example III The procedure of Example I is repeated excepting cadmium sulfide is substituted for the cadmium sulfoselenide photoconductive material. In this instance, runs as long as about 50,000 prints are obtained.

Example IV Example V The procedure of Example IV is repeated excepting zinc sulfide is substituted for the cadmium sulfoselenide photoconductive material. Formaldehyde is substituted for the carboxymethyl cellulose of the fountain solution. Runs as long as about 53,000 prints are obtained.

Example VI The process of Example V is repeated excepting copper phthalocyanine is substituted for the zinc sulfide photoconductive material. Runs as long as about 53,000 prints or a plate equivalent to that of Example V is obtained.

Example VII A glass frit, Coming 1970, an electroluminescent phosphor imbedding glass, available from the Corning Glass Co. is utilized as the binder in this test run.

The remainder of the procedure is identical to that of Example I. The runs made with this lithographic plate produced about 48,000 prints.

Example VIII The process of Example VII is repeated excepting zinc oxide is substituted for the cadmium sulfoselenide photoconductive material. Also, formaldehyde is substituted for the carboxymethyl cellulose of the fountain solution. Runs of about 50,000 prints are obtained.

Example IX The procedure of Example VII is repeated excepting zinc sulfoselenide is substituted for the cadmium sulfoselenide photoconductive material. Runs of about 49,000 prints are obtained.

Example X The process of Example I is repeated excepting Corning No. 1971 electroluminescent phosphor i-mbedding glass, commercially available from the Corning Glass Company, is substituted for the glass frit of Example I. The remainder of the procedure being the same runs as long as about 45,000 prints are obtained.

Example XI The process of Example X is repeated excepting zinc oxide is substituted for the cadmium sulfoselenide photoconductive material. Runs of about 50,000 prints are obtained.

Example XII The procedure of Example X is repeated excepting zinc sulfoselenide is substituted for the cadmium sulfoselenide photoconductive material. In this instance, formaldehyde was substituted for the carboxymethyl cellulose of the fountain solution. The lithographic plate in this instance produces successful runs as long as about 52,000 prints.

Example XIII The process of Example X is repeated, however, in this instance zinc sulfide is substituted for the cadmium sulfoselenide photoconductive material. The lithographic plate in this case produced runs as long as about 50,000 prints.

Example XIV The glass frit fromulation utilized in this test run is Du Pont L232, a procelain enamel frit, commercially available from the Du Pont Co. The remainder of the procedure was similar to that of Example 1. Runs as long as about 48,000 prints are obtained.

Example XV A glass frit formulation corresponding to Harshaw Flux AG 850, commercially available from the Harshaw Chemical Co., is used in the lithographic plate of this example. The remainder of the procedure is similar to that of Example I. After the application of a lithographic ink, runs of about 50,000 prints are obtained.

Example XVI Utilizing a glass frit formulation of Example XV, a lithographic printing plate is prepared according to the procedure of ExampleI. In this instance, however, a Z1110 oxide photoconductive material is substituted for the cadmium .sulfoselenide. In this instance runs of about 50,000 prints are obtained.

Example XVII The procedure of Example XV is repeated excepting zinc sulfoselenide is substituted for the cadmium sulfoselenide photoconductive material. In this instance, however, formaldehyde is substituted for the carboxymethyl cellulose of the fountain solution. The lithographic plate of this example produces runs up to about 53,000 prints.

Example XVIII The procedure of Example XV is repeated excepting zinc selenide is substituted for the cadmium sulfoselenide photoconductive material. In this instance, formaldehyde is substituted for the carboxymethyl cellulose of the fountain solution. The lithographic plate of this example produces runs up to about 53,000 prints.

Example XIX A glass frit formulation corresponding to Harshaw Flux AG 862, commercially available from the Harshaw Chemical Co., is used in the lithographic plate of this example. The remainder of the procedure is similar to that of Example I. After the application of a lithographic ink runs approximating 50,000 prints are obtained.

Example XX The procedure of Example XIX is repeated excepting zinc oxide is substituted for the cadmium sulfoselenide photoconductive material. The resulting lithographic plate produced up to about 50,000 prints.

Example XXI The procedure of Example XIX is repeated excepting zinc cadmium sulfide is substituted for the cadmium sulfoselenide photoconductive material. Runs up to about 50,000 prints are obtained.

Example XXII The procedure of Example XIX is repeated excepting zinc cadmium selenide is substituted for the cadmium sulfoselenide photoconductive material. The lithographic plate obtained is comparable to that of Example XIX.

Example XXIII A lithographic plate is prepared according to the procedure of Example I. After the plate is used to make the prints desired, the imaged areas of the plate are cleaned with methyl ethyl ketone in order to remove the developed image. The lithographic plate is then regenerated by heating to about 450 F. for approximately two minutes. The plate is recharged and reimaged according to the procedure of Example I with the resulting lithographic plate producing up to 45,000 prints.

10 Example XXIV In order to further demonstrate the reusability of the lithographic plates of this invention the printing plate of Example IV was treated in a manner similar to that of Example XXIII. It was found as in the preceding example that the rejuvenated lithographic plate could be reimaged and reused in lithography, the resulting plate producing up to about 48,000 prints.

Although the present examples were very specific in terms of conditions and materials used, any of the above listed typical materials 'may be substituted when suitable in the above examples with similar results. In addition to the steps used to prepare the lithographic plate of the present invention, other steps or modifications may be used, if desirable. For example, the fountain solution and lithographic ink may be applied in a single step to the surface of the plate. In addition, other materials may be incorporated in the developer, ink, fountain solution photoconductive material or xerographic plate which will enhance, synergize or otherwise desirably affect the properties of these materials for the present use. For example, the spectral sensitivity of the plates prepared in accordance with the present invention may be modified by incorporating photosensitizing dyes therein.

Anyone skilled in the art will have other modifications occur to him based on the teaching of the present invention. These modifications are intended to be encompassed within the scope of this invention.

What is claimed is:

1. A method of making multiple copies from a lithographic master which comprises forming an electrostatic latent image on the surface of a hydrophilic glass ph0t0 conductive plate, said plate comprising a conductive receiving substrate having fixed to at least one surface thereof a glass photoconductive insulating layer, developing said image with a hydrophobic developer so as to form image areas that are hydrophobic and non-image areas that are hydrophilic, applying to the imaged surface of said hydrophilic glass photoconductive plate a lithographic ink, said ink being distributed thereon conforming to said hydrophobic image in an image'wise configuration, contacting said inked surface with a transfer sheet to thereby affect the transfer of copies of said image to said sheet and repeating the inking and contacting steps until the desired number of copies are produced.

2. The process as defined in claim 1 wherein said glass photoconductive insulating layer comprises a photoconductive material dispersed in a glass binder insulating composition.

3. The process as defined in claim 2 wherein said photoconductive material comprises an inorganic photoconductive material.

4. The process as disclosed in claim 2 wherein said photoconductive material comprises a phthalocyanine pigment.

5. A method of making multiple copies from a xerographic image which comprises:

(a) forming an electrostatic latent image on the surface of a hydrophilic glass photoconductive plate, said plate comprising a conductive receiving substrate having fixed to at least one surface thereof a photoconductive insulating layer comprising a photoconductive material dispersed in a glass binder composition,

(b) developing said image with a hydrophobic developer so as to form image areas hydrophobic in nature and non-image areas hydrophilic in nature,

(c) applying to the imaged surface of said hydrophilic glass photoconductive plate a lithographic ink in such a manner that said ink is distributed thereon conforming to said hydrophobic image in an imagewise configuration,

(d) contacting said inked surface with an off-set blanket to thereby affect the transfer of an imprint of said image to said blanket,

(e) contacting said imaged oil-set blanket with a transfer sheet, and

(f) repeating steps through e until the desired copies are produced.

6. The process as disclosed in claim wherein said photoconductive material comprises an inorganic photoconductive material.

7. The process as disclosed in claim 6 wherein said inorganic photoconductive material is selected from at least one member of the group consisting of zinc oxide, zinc sulfide, zinc cadmium sulfide, cadmium selenide, cadmium sulfide, cadmium sulfoselenide, zinc cadmium selenide and zinc sulfoselenide.

8. The process as disclosed in claim 5 wherein said photoconductive material comprises a phthalocyanine pigment.

9. A method of making multiple copies from a lithographic master which comprises forming an electrostatic latent image on the surface of a hydrophilic glass photoconductive plate, said plate comprising a conductive receiving substrate having fixed to at least one surface thereof a glass photoconductive insulating layer comprising an inorganic photoconductive material selected from at least one member of the group consisting of zinc oxide, zinc sulfide, zinc cadmium-sulfide, cadmium selenide, cadmium sulfide, cadmium sulfoselenide, zinc cadmium selenide and zinc sulfoselenide dispersed in a glass binder insulating composition, developing said image with a hydrophobic developer so as to form image areas which are hydrophobic in nature and non-image areas hydrophilic in nature, applying to the surface of said hydrophilic glass photoconductive plate a lithographic ink, said ink being distributed thereon conforming to said hydrophobic image in an imagewise configuration, contacting said inked surface with a transfer sheet to thereby affect the transfer of copies of said image to said sheet and repeating the inking and contacting steps until the desired number of copies are produced.

10. The process as disclosed in claim 9 wherein said receiving substrate comprises a metallic material having an electrical resistance less than the photoconductive layer.

References Cited UNITED STATES PATENTS 1,803,519 5/1931 Zeh 101149.2 2,993,787 7/1961 Sugarman. 3,151,982 10/1964 Corrsin 96l.8

FOREIGN PATENTS 1,258,844 3/1961 France.

DAVID KLEIN, Primary Examiner US. Cl. X.R. 

